Strapping joint and method for forming same

ABSTRACT

An improved joint and method for providing the improved joint structure in joined or fused thermoplastic strap positioned about a package or article is disclosed. The improved joint includes means for increasing the peel separation resistance of overlapping portions of the joint above the peel separation resistance provided by a weldment between the overlapping portions providing for a joint strength that approaches the tensile strength of the strap material or weldment.

This is a continuation of Ser. No. 07/347,569 filed May 5, 1989, nowU.S. Pat. No. 5,006,385, which is a continuation-in-part of Ser. No.07/223,482 filed July 22, 1988 abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to an improved strapping joint and themethod for providing the improved joint. More specifically the inventionprovides an improved joint structure in sealless joints of thermoplasticstrapping.

In the recent past, Signode Corporation, the assignee of the entireinterest in the present application, has developed several processes,tools and power strapping machines for joining the overlapping endportions of a tensioned thermoplastic strap loop by friction-fusiontechniques, and these methods and apparatuses are typified by thosedisclosed and claimed in U.S. Pat. Nos. 3,331,312, 3,442,203, 3,442,732,3,442,733, 3,442,734, 3,442,735, 3,494,280, 3,586,572, 3,669,799,3,679,519 and 3,718,526.

The thermoplastic strap material used in the practice of the inventionsdisclosed in the above named patents have conventionally been orientednylon, polypropylene and polyester. Straps formed from such materialsare fabricated by generally similar techniques, as is disclosed, forexample, in U.S. Pat. No. 3,394,045. According to this patent,polypropylene sheet material, which is subsequently sliced into straps,is stretched to obtain planar orientation of the micromolecular chains.The molecular orientation of the polypropylene resin in the sheetsubstantially increases the tensile strength along the lines oforientation. However, this also renders the opposing surfaces of thesheet susceptible to abrasion which could ultimately interfere with itsuse in a strapping apparatus. To overcome this, the surfaces of thesheet are subjected to flash-heating to effect fusion of the outeropposing layers but insufficient to cause fusion in the entire thicknessof the sheet. The flash heating acts to smooth the opposing surfacelayers, but does not disrupt the molecular orientation of the rest ofthe sheet material so that the high tensile strength is retained. Theformed sheet material is then sliced to produce straps. Slicing is donegenerally parallel to the molecular lines of orientation so that thestraps have high tensile strength along their length. This makesexcellent straps for use in the packaging of materials where a strap isdrawn tightly around a package with overlapping end portions secured toeach other in the form of a ligature. The overlapping end portions canbe secured by securing means, which include strap seal, adhesive orweldment such as friction fusing or thermoweld.

One of the characteristics of fused joints formed in accordance with theteachings of the above patents is their exceptional tensile strength(i.e., resistance to forces applied in the direction of the length ofthe strap). A further characteristic of such joints is that the peelstrength (i.e., resistance to forces applied normally to the length ofthe strap) is relatively low. Although the low peel strength offriction-fused joints is very useful in enabling the strap to be removedfrom the strapped article, on occasion it may also result inunintentional opening of the joint. For example, if the end of theoutermost strap portion is snagged in the handling of the article thejoint could peel open. It has also been found that peel forces are moreapparent for thicker strap as opposed to thinner strap material.However, it is also an ongoing problem that some weld joints are subjectto premature failure.

SUMMARY OF THE INVENTION

The present invention is directed to an improved strap joint havingimproved joint strength, through position control of the overlapping endportions of a thermoplastic strap on opposite sides of a joint weldmentwhich are proximate the inner and outer surfaces of the overlapping endportions of the strap and to a method for forming the improved joint.

According to the invention, thermoplastic strap from a strap supply ispassed around an article; drawn tightly into a loop with a leading endportion adjacent the article and a trailing end portion outermost fromthe article overlapping; the outer trailing portion of the strap is thencut; and, the overlapping portions are joined to one another, forexample, by a resolidified thin layer of melted material from each ofthe overlapping strap portions provided by techniques known in the art,such as friction fusion or hot-knife methods.

The friction-fusion process may be practiced by positioning theoverlapping portions of a thermoplastic strap between a pair of weldingjaws having opposing strap-gripping surfaces. The gripping surfacesoscillate at a high frequency to effect a weldment.

The improved strap joint according to the invention inhibits peel byincreasing the resistance of the overlapping portions of the strap onopposite sides of the weldment at the facing inner and outer surface ofthe strap to separation when the weldment is in tension, and thus thestrap joint or weld can more consistently accommodate a tensile load onthe strap approximately equivalent to the weld or strap tensilestrength.

The basic manufacturing steps of the invention include:

1) providing a strap from a source of strapping about an article orarticles with portions overlapping;

2) cutting the strap to length;

3) forming a joint at the overlapping portions of the thermoplasticstrap surrounding the article by providing a weldment between theoverlapping portions along at least a portion of the length of theoverlapping portions;

4) maintaining the joint in a constrained position to provide animproved and more consistent joint strength characteristic duringsubsequent tensile loading of the overlapping portions; and,

5) providing means for increasing the peel separation of the overlappingportions at opposite sides of the weldment proximate the facing innerand outer surfaces of the overlapping portions when the overlappingportions are tensile loaded

BRIEF DESCRIPTION OF THE DRAWING

In the several figures of the drawing, like reference numerals identifylike components, and in those figures:

FIG. 1 is a schematic side-view illustration of a strap about arepresentative article;

FIG. 2 illustrates an enlarged view of the overlapping portions of astrap joint as shown in FIG. 1;

FIG. 3 illustrates a schematic weld joint between overlapping portionsof thermoplastic strapping;

FIG. 4 is a schematic illustration of a first embodiment of an improvedjoint structure according to the invention;

FIG. 5 is an alternative embodiment of an improved joint structureaccording to the invention;

FIG. 6 illustrates a second alternative embodiment of an improved jointstructure according to the invention; and

FIG. 7 illustrates a still further embodiment of the improved jointstructure according to the invention

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method of conveniently and consistentlyproviding for a weld joint of thermoplastic strapping generally formedabout an article such as a package. Hereinafter, whenever referred to,the term opposite sides of the weldment means those sides of theweldment which are proximate the facing inner and outer surfaces of theoverlapping portions of the strap respectively. The strapping isprovided with a means for maintaining overlapping portions of the strapon opposite sides of the weldment portion of the joint in a relativeproximate relationship and substantially non-separable in peel whentensioned such that the total joint strength consistently approaches thestrap material or weldment tensile strength. More specifically, anyunwelded length of the overlapping portions of the strap extendinglongitudinally beyond either end of the weldment are maintained inrelative proximity to the mating strap inner and outer surface by themeans which resists separation of the overlapping portions on theopposite sides of the weldment. Means according to the invention asdescribed below which resists separation of the overlapping portions ofthe joint on opposite sides of the weldment prevents a conditionreferred to as "peel". The thermoplastic strap material may be anyplastic-like material such as polypropylene or polyester, as known inthe art but preferably is a material having a high I-V (intrinsicviscosity) such as polyethylene terephthalate having an intrinsicviscosity of at least 0.8.

In the drawings, FIG. 1 illustrates a schematic view of a package 10with a plastic strap 12 positioned or loosely draped about the packagefor securing it generally in anticipation of storage or shipmentthereof. The strapped package or strapped material 10 of FIG. 1includes: an illustration of a weld joint in the form of a weldment 14along at least a portion of the surfaces of overlapping portions 11, 15of the strap; and, an illustration of a knife means 20 for severing thestrap material 12 about the package 10 from a source of strap 13. Asshown in FIG. 3, a trailing end portion 16 and a leading end portion 18of the overlapping portions 11, 15 of the strap refer to the terminusportions of strap 12 after it has been cut. As shown, the end portions16 and 18 refer to unwelded overlapping strap lengths extending beyondthe opposite ends 31, 33 of the weldment 14 and not just to the terminuspoints of the strap. As noted, the weldment 14 extends along at least aportion of the length of the overlapping portions, meaning that theweldment can, depending on the welding technique utilized, extend alongeither the entire length of the overlapping portions 11, 15 or alongonly a portion leaving at least one welded trailing or leading endportion 16, 18 as shown in FIG. 3.

FIG. 3 is an enlarged view of the overlapped trailing end portion 16with first terminus 17 and the leading end portion 18 with secondterminus 19 of strap 12. Strap 12 is shown as a thin walled,longitudinally extending material having an inner planar surface 22 toone side of the weldment which engages against the article 10 and anouter planar surface 24 on the opposite side of the weldment disposedouter-most from the article which directional outer and inner referenceindicates position in the several figures relative to the article. Theoverlapping portions 11, 15 are illustrated FIGS. 1 and 2 in theirseparated position prior to joining by any means known in the art, suchas friction welding, adhesive or thermowelding. The overlapping portionsare joined by the weldment 14 between the inner and outer surfaces, asnoted along at least a portion of the length of the overlapping portionswith any unwelded portions 16, 18 disposed on opposite ends 31, 33 ofthe weldment as shown in FIG. 3. As noted, the weldment 14 has a firstend 31 in proximity to the strap outer trailing portion 16 and a secondend 33 in proximity to the strap inner leading end portion 18. A tensileload is generally applied to the strap 12 on opposite sides of the jointafter welding and the strength of the joint is noted by its resistanceto separation in the weld joint under the tensile load. The tensile loadstrains in opposite directions on either side of weldment 14, asillustrated in FIG. 3 by arrows T1 and T2. This loading on either sideof the weldment 14 may lead to strap separation at tensile loadssignificantly lower than either the tensile strength of the strap, or ofthe weld. The strap separation may be caused by peel, that is where theoverlapping strap portions 11, 15 are peeled away from each other bygenerally vertically, oppositely-directed forces F1, F2 from bendingmoments acting on the overlapping portions 11, 15 on opposite sides ofthe weldment. These forces can cause separation or peel of the weldmentat tensile forces significantly less than the tensile strength of eitherthe weldment 14 or the strap. The present invention provides means formaintaining the overlapping portions 11, 15 including any unweldedleading overlapping end portion 18 and trailing overlapping end portion16 in proximity to and substantially non-separated from thecorresponding adjacent overlapping portion when the joint is in tension,which means operates to increase the peel separation resistance of theoverlapping portions 11, 15 including portions 16, 18 on opposite sidesof the weldment and thus reduces the likelihood of premature weldseparation and strap failure when the joint is tensioned. The means forincreasing the peel separation resistance is further shown by theseveral embodiments illustrated in FIGS. 4-7.

A first embodiment of the present invention is shown in FIG. 4. In thisembodiment, the strap 12 is applied about a package 10. The overlappingstrap portions 11, 15, are joined or welded together by a weldment 14,by means known in the art. As illustrated, first separation resistingmeans 26 and second separation resisting means 28 are provided about thestrap overlapping portions 16 and 18, which physically or mechanicallyresists vertical forces acting on the overlapping portions 16, 18 onopposite sides of the weldment 14 during tensile loading of the strap 12and joint. The resisting means 26 and 28, for example, may be wire, tapeor spot weldments at the outer edges of strap 12. Spot weldments 27 and29 in FIG. 7 illustrate physically securing the outer and inneroverlapping portions 16, 18 on the opposite ends of the weldment 14together which increase the resistance of the overlapping portions 11,15 on opposite sides of the weldment to peel separation. Thereafter, theapplied tensile load-to-failure has been found to consistently approachthe tensile strength of strap 12. Strap 12 is thus allowed to realizethe full tensile load at the weldment without concern of failure frompeel at the weldment.

A second, or alternative, embodiment of the present invention isillustrated in FIG. 5 wherein the overlapping portions 11, 15 have beenoverlapped and welded. Weldment 14 has been appropriately provided alongat least a portion of the length of the inner and outer surfaces 22, 24with the trailing end overlapping portion 16 and the leading endoverlapping portion 18 on opposite ends of the weldment 14 however, asnoted, depending on the welding technique used, the weldment can extendalong the entirety of the overlapping portions. Prior to tensileloading, the strap is bent deformed or shaped in a Z-like form eitherduring or immediately after the welding process by laterally relativelydisplacing the strap such that the strap portions 16, 18 that extendbeyond the ends of the weldment lie on opposite sides of the thenangularly disposed connecting plane of the weldment and preferably tolie in parallel planes with each other. Strap 12 in the illustratedembodiment generally defines a plane with a longitudinal axis, 21 inFIG. 3, along the mating inner and outer surfaces 22, 24 of strap 12 asshown in FIG. 3. In FIG. 3 the overlapping portion 16 prior to tensileloading is normally disposed with inner and outer surfaces 22, 24co-planer but in the absence of the present invention the outer strapportion at side 16 would generally be displaced in a direction normallyaway from outer surface 24 upon tensile loading of strap 12. Similarly,the overlapping portions at side 18 are shown in their normal positionwherein inner and outer surfaces 22, 24 are also co-planer on that sideof the weldment prior to tensile loading but which inner overlappingstrap portion at side 18 would generally be displaced in a directionnormally away from the plane of inner surface 22 after tensile loadingof strap 12. It has been found that, by forming the overlapping portionseither simultaneously with or after forming the weldment 14 so as to beon opposite sides of the plane of the weldment, when the strap 12 isallowed to elongate under a tensile load T1, T2, the outer portion ofthe strap at the side 16 due to a normal force acting on the outerportion of the strap at side 16 and the inner overlapping portion of thestrap at side 18 moves to contact or at least resist separation from theinner surface 22 of the adjoining overlapping portion at side 18 due toan oppositely directed normal force acting on the inner portion of thestrap at side 18. The overlapping portions 16, 18 on opposite ends ofthe weldment are thus maintained substantially non-separable undertension and the resistance of the overlapping portions 11, 15 to peelseparate is increased.

A third, or second alternative, embodiment is illustrated in FIG. 6wherein the outer overlapping strap portion 11 is curved radiallyoutward or convex relative to the outer strap surface 24 on oppositesides of the weldment. The deformation, or forming, of the outeroverlapping strap portion is provided near weldment 14 prior toresolidification of the molten or semi-plastic material state atweldment 14. This deformation, while the weldment or joining materialbetween the overlapped strap portions is in its semi-molten state,stretches or elongates the molten material normal to the surface at theends of the weld. This semi-molten plastic material resolidifies andgenerally forms a honeycomb shape, stretched plastic orstalactite-stalagmite structure 30 at either ends 31, 33 of theweldment. This structure provides for increasing the resistance to peelseparation of the overlapping portions 11, 15 at both ends 31, 33 of theweldment during strap 12 tensile loading. The predetermined deformationstretched plastic structure should provide for a slight separationbetween the strap outer surface 24 and the strap inner surface 22 ofpreferably at least equal to about one strap thickness.

While it has been noted, that the weldment portion 14 of the joint canbe effected by well known means, the preferred weldment is one createdby friction fusing wherein at least one of the overlapping strapportion, for example the outer portion, is rapidly transverselyoscillated relative to the adjoining overlapping strap portion, forexample the inner portion, while the two portions are pressed togetherso as to create a sense-molten state of the plastic which when allowedto resolidify joins to form the weldment. Known friction fusing methodsand the tool for carrying out the known methods allow the semi-moltenplastic to resolidify during a period wherein the relative oscillatorymotion of the overlapping strap porions is damping out under opposingfrictional and inertial forces principally from the tool. This slowdamping out during resolidification disrupts the integrity of theplastic by introducing voids or inclusions into the weldment.

In contrast to these known methods, the preferred method for forming thefriction fusion type weldment of the present invention is preferablymade by abruptly stopping the relative motion of the overlapping strapportions when the semi-molten state of the plastic is reached so thatthe resolidification of the plastic will occur preferably with no or atleast as little physical disruption as possible. Accordingly, theweldment portion of the total joint is stronger than known joints whichin combination with the means described herein providing for resistanceof the overlapping portions of the joint on opposite sides of theweldment to separate, and in particular the means disclosed in FIG. 5,provides a significantly stronger total joint than known joints.

It has been found that these various structures or embodiments providefor increasing the resistance of the overlapping portion of the strapjoint on opposite sides of the joint weldment to peel separation abovethat heretofore provided solely by the weldment itself during theapplication of the tensile load to the strap and are effective inpreventing peel separation of the weldment ends. The maintenance of thepositional relationship of the overlapping portions of the strap jointon opposite sides of the joint weldment inherently provides therequisite position for enhancing tensile loading of the strap, and thusthe improvement of the joint and the joint integrity.

While this invention has been described in connection with certainspecific embodiments hereof, it is to be understood that this is by wayof illustration and not by way of limitation; and the scope of theappended claims should be construed as broadly as the prior art willpermit.

We claim:
 1. A method of forming a joint between overlapping portions ofa thermoplastic strap surrounding an article, the stepscomprising:surrounding the article with an untensioned length ofthermoplastic strap with an inner surface of the strap facing thearticle and an outer surface of the strap facing away from the articleand overlapping portions of the strap being untensioned; joining theoverlapping portions of the strap by providing a weldment between saidinner and outer surfaces of the strap along at least a portion of theoverlapping portions of the strap while the overlapping portions and theportion of the strap surrounding the article are both untensioned; andproviding means for increasing resistance of said overlapping portionsof the strap on opposite sides of said weldment to peel separate above apeel separation resistance provided by said weldment when saidoverlapping portions of the strap at said weldment are put in tensionincluding providing means for creating opposing forces in response tothe tension on said overlapping portions, which opposing forces act onsaid overlapping portions on opposite sides of said weldment indirections urging said overlapping portions on each of said oppositesides of said weldment together.